1. Field of the Invention
The present invention relates to a solar cell module and, more particularly, to a solar cell module comprising a reinforcing member (substrate) such as a metal sheet on the non-light-receiving surface side (hereinafter referred to as "bottom surface") of a photovoltaic device, and to the reinforcing member.
2. Related Background Art
Increase in consciousness of environmental issues is spreading on a worldwide basis in recent years. Among others, concern is deepening about the warming phenomenon of Earth due to CO.sub.2 emission, so that desires for clean energy are becoming stronger and stronger. Solar cells can be said to be promising as a clean energy source at present because of their safety nature and ease to handle.
There are various forms of solar cells. Typical examples thereof are as follows.
(1) Crystalline silicon solar cells PA1 (2) Polycrystalline silicon solar cells PA1 (3) Amorphous silicon solar cells PA1 (4) Copper indium selenide solar cells PA1 (5) Compound semiconductor solar cells
Among these, research and development are recently active in various fields on the thin-film crystalline silicon solar cells, compound semiconductor solar cells, and amorphous silicon solar cells because increase of the area thereof can be achieved at relatively low cost.
Further, among these solar cells, the thin-film solar cells, typified by the amorphous silicon solar cells in which silicon is deposited on a conductive metal substrate and a transparent conductive layer is formed thereon, are considered to be promising as a future module form because they are lightweight and have high impact resistance and flexibility. Unlike the case where silicon is deposited on a glass substrate, it is, however, necessary to cover the light-incidence-side surface by a transparent covering member to protect the solar cell. The most general method is a method for using glass on the topmost surface and bonding the glass to the solar cell device with an encapsulating resin. Since glass has excellent weather resistance and is impervious to moisture, it can be said to be one of the most excellent materials for the member for covering the photovoltaic device of semiconductor. This is the reason why the most solar cell modules employ glass as a covering member for the topmost surface.
The glass covering member, however, has the problems of 1) being heavy, 2) incapability of being curved, 3) being weak against impact, and 4) costing high. These problems hinder utilization of the advantages such as the lightweight, high impact resistance, and flexibility of the thin-film solar cells.
There have, therefore, been suggested heretofore lightweight and flexible solar cell modules taking advantage of the features of the thin-film solar cells by using a transparent fluoride polymer film, such as a fluororesin film, as a surface covering member of the topmost surface and a variety of thermoplastic, transparent, organic resins as an encapsulating resin inside thereof. Reasons why these materials have been used are, for example; 1) that the fluoride polymer has so high weather resistance and water repellency as to reduce decrease in conversion efficiency of solar cell module due to decrease in optical transmittance caused by yellowing or whitening resulting from deterioration of the resin or by contamination of the surface; 2) that the thermoplastic, transparent resins are cheap and can be used in large volume as an encapsulating material for protecting the internal photovoltaic device. On the solar cell devices there are normally provided various collector electrodes for efficiently leading out generated power and metal members for series or parallel connection between devices. The thermoplastic, transparent, organic resins also have such an effect that mounted members including the electrodes, the metal members, etc. are also encapsulated, so as to even roughness on the device surface to make the covering member surface smooth.
The solar cell modules covered by such films are flexible, but naturally have lower mechanical rigidity than in the case using glass.
In order to improve it, it is thus common practice to stick one selected from various reinforcing members (substrates) through an adhesive layer to the bottom surface. Normally, the bottom reinforcing member (bottom reinforcing plate) is a steel sheet or a plastic sheet with high rigidity. Development is also active of a roof-integrated solar cell module taking advantage of the flexibility of the solar cells coated with the film. In this case, the photovoltaic device is stuck through the adhesive layer to a steel plate for roof. In other words, the steel plate for roof functions as a reinforcing plate.
FIG. 1 illustrates an example of such a solar cell module. In FIG. 1, reference numeral 103 designates a transparent member made of a fluoride polymer thin-film layer, 102 a filler made of a thermoplastic, transparent, organic resin, 101 a photovoltaic device, 104 an insulation sheet, 105 an adhesive, and 106 a reinforcing plate. In this example the bottom encapsulating material is the same as the organic resin on the light-receiving surface side.
More specifically, the transparent member 103 is a fluororesin film such as an ETFE (ethylene-tetrafluoroethylene copolymer) film, a PVF (polyvinyl fluoride) film, or a PVDF (polyvinylidene fluoride) film, the filler 102 is one selected from EVA (ethylene-vinyl acetate copolymer), butyral resin, etc., the insulation sheet 104 is one selected from various organic resin films including a nylon film, a PVF film, a polyethylene terephthalate (PET) film, and a polyethylene film, the adhesive 105 is one selected from hot melt adhesives such as EVA (ethylene-vinyl acetate copolymer) or butyral resin, epoxy resin, silicone resin, and so on, and the reinforcing plate 106 is one selected from a galvanized steel sheet, a Galvalume sheet, a stainless steel sheet, an acrylic sheet, a polycarbonate sheet, a fiber-reinforced plastic (FRP) sheet, and so on.
In this example, the filler 102 functions as an adhesive between the photovoltaic device 101 and the transparent member 103 and between the photovoltaic device 101 and the insulation sheet 104, and as a filler for protecting the solar cell from scratching and impact from the outside.
Incidentally, since the solar cell modules are used under severe outdoor circumstances, the components thereof are required to have high weather resistance and durability. The reinforcing plate is not an exception, either, and it is processed by a variety of means for enhancing the durability. For example, in the case of the reinforcing plate of steel, in order to suppress corrosion thereof, the surface is subject to corrosion-resistant plating such as zinc plating, aluminum plating, or zinc-aluminum alloy plating, or the surface is coated with a weather-resistant paint containing a main component selected from fluororesin, silicone resin, polyester resin, acrylic resin, epoxy resin, and so on. In the case of the reinforcing plate of plastic, a plastic plate with high durability is selected, such as the fiber-reinforced plastic (FRP).
When the reinforcing plate is provided with the resin layer to enhance the weather resistance and durability and if the resin layer has some thickness taking detachment of the resin surface layer due to deterioration into consideration, cracks will appear easier and sometimes pose a problem that the resin layer becomes more likely to be peeled off from the reinforcing plate, however. Accordingly, use of the reinforcing plate with the thick organic resin layer on the surface on the solar cell device side will sometimes result in peeling off of the organic resin layer from the reinforcing plate, i.e., peeling off of the photovoltaic device from the reinforcing plate in long-term outdoor exposure or in various accelerated degrading tests.
Fluorine-based paints and silicone-based paints, which are typical weather-resistant paints for the covering member of the surface of steel plate, have stable chemical bonding and low chemical activity of surface, i.e., large water repellency. In general, they often have poor adhesion to the adhesive resin or often lack sufficiently strong adhesive strength. Accordingly, use of the reinforcing plate provided with a fluororesin layer or a silicone resin layer on the surface on the solar cell device side will sometimes result in peeling off of the adhesive resin from the reinforcing plate, i.e., peeling off of the photovoltaic device from the reinforcing plate in the long-term outdoor exposure or in the various accelerated degrading tests.
Further, it was common practice heretofore to use the reinforcing plate without especially distinguishing the top from the bottom and enhance the weather resistance and durability on the both surfaces. Therefore, the high cost of the reinforcing plate was another problem.
These problems should be taken into more consideration in the case of the roof-integrated solar cell modules of the structure in which the reinforcing plate projects greatly outside the photovoltaic device. FIG. 2 illustrates the sectional view showing a typical roof-integrated solar cell module. In FIG. 2, reference numeral 201 denotes a solar cell device, 202 an encapsulating material, 203 a transparent member, 204 an insulation sheet, 205 an adhesive, and 206 a reinforcing plate.
In the roof-integrated solar cell module, the encapsulating material 202 and top surface member 203 are formed to project out on the reinforcing plate 206. Further, projecting portions of the reinforcing plate 206 are bent so as to match the roof shape, for example, as shown in FIG. 2. In this case, the reinforcing plate 206 is not only in contact with the adhesive 205 under the photovoltaic device 201, but also in contact with the encapsulating material 202. In addition, parts of the portions in contact with the encapsulating material 202 are in a stress-accumulated state due to bending. When the module in this state is installed outdoor over a long period, more peeling off could occur between the reinforcing plate 206 and the encapsulating material 202 in the bent portions in addition to the aforementioned peeling off of device. This sometimes appears very outstanding if the high-weather-resistant resin layer such as the fluororesin or the silicone resin is provided on the device side of the reinforcing plate 206.